X-ray CT imaging technology has been widely used in clinical therapy, security inspection and industrial nondestructive testing and other fields. Currently, mainstream CT technology uses a single X-ray energy spectrum, and an energy integral detector is utilized to obtain a distribution image of linear attenuation coefficient inside the scanning object by using CT reconstruction method. The distribution image of linear attenuation coefficient is a grayscale image reflecting X-ray absorption intensity of the material and cannot meet a requirement of accurate identification of the material for the clinical therapy and security inspection. The emergence of dual-energy X-ray CT technology provides a more powerful way to achieve accurate material identification. The dual-energy CT may reconstruct to get distribution images of an atomic number Z and electron density ρe the object being scanned by collecting projection data under two different X-ray spectra which is usually recorded as low-energy data and high-energy data and calculating through a special dual-energy CT material decomposition algorithm.
As an attenuation coefficient of the material varies with change of the photon energy, the most common X-ray source in X-ray CT is X-ray machine or accelerator, the X-ray emitted from which has a wide spectrum and is not monochrome. Therefore, a projection model of a traditional CT should be nonlinear. The equivalent attenuation coefficient is obtained by directly reconstructing the projection data, which is an average value of the linear attenuation coefficient function commonly affected by the radiation source spectrum and the radiated object, and does not have a clear physical meaning and may only provide structure information of the radiated object. In contrast, the dual-energy CT obtains two sets of projection data by utilizing two different X-ray spectrums and implements material identification through a special dual-energy CT material decomposition method.
The dual-energy CT decomposition algorithm may be divided into three categories: a projection domain pre-processing method, an image domain post-processing method and an iterative method. The projection domain pre-processing method considers that the linear attenuation coefficient function may be decomposed into a linear combination of two known basic functions which only take energy as variables. For each of ray paths, an integral value of the linear combination on the ray path is calculated by using the dual-energy projection value on the pat, and then the combined coefficients of each point are obtained according to the traditional CT image reconstruction algorithm, which corresponds to feature quantity of the linear attenuation coefficient function at each point. The linear attenuation coefficient function or substance information (atomic number and electron density) of such a point may be determined according to the feature quantity. The projection domain pre-processing method may effectively remove hardened artifacts, but requires dual-projection path matching. The image domain post-processing method firstly reconstructs the projection data of the dual-energy CT respectively to obtain reconstructed CT images at two different energy spectrums and then compares the reconstruction value of each pixel and the basis material image to complete the material decomposition. The image domain post-processing method is simple to implement, but cannot remove hardened artifacts. The iterative method is to integrate CT image reconstruction and material decomposition into one mathematical model. The iterative method may be used to complete the final material decomposition, and the iterative method may also add other priori constraints, such as statistical noise model, and may optimize the iterative solution with maximum likelihood (ML), Maximum expectation (EM) or maximum a posteriori (MAP). The advantage of the iterative method is that it may better suppress noise, but its computational process is more complex and time consuming.
In the dual-energy X-ray CT imaging, the theoretical basis for the three kinds of material decomposition methods as mentioned above is interaction of X-ray and the substance. Considering that the X-ray energy applied in fields such as clinical therapy, security inspection, industrial non-destructive testing and custom smuggling detection ranges between 10 keV˜10 MeV, there are four possible interactions: Rayleigh scattering, photoelectric effect, Compton scattering and electron pairing effect, which may completely describe the attenuation coefficient function of many substances. In practice, since the dual-energy CT only collects projection data of the high and low energy, the dual-energy CT may only use the mathematical model of two interactions to decompose the material, and ignore the effects of other interactions. For example, in the low-energy imaging, only the photoelectric effect and Compton scattering are considered, while in the high-energy imaging only the Compton scattering and electron pairing effects are considered. Although these approximations enable the dual-energy CT imaging to decompose the material, it will inevitably bring some error, and in some cases these error becomes nonignorable. Especially in the high-energy dual-energy CT imaging, such as the detection of radioactive nuclear materials, large van container inspection system, air box inspection system, and large metal work piece nondestructive testing and so on, the cross-section of the electron pairing effect is not much larger than that of the photoelectric effect. If only one method is taken into consideration, it will have to bring obvious errors and different effects on the decomposition of materials have to be considered to effectively improve the effect of high-energy dual-energy CT imaging.